In the capillary electrophoresis method, it has been carried out in the past to use one kind of internal standard substance and specify an approximate position of an analyte using a peak thereof as a tip marker (see Patent Literature 1). However, when only one kind of internal standard substance is used, positional relations among peaks of an internal standard substance and an analyte may be varied by influence of measurement conditions and impurities in a sample. Therefore, when position of an analyte is required to be specified with good accuracy, a molecular weight marker has been used.
When an analyte is DNA or RNA, it has been also known that measurement is carried out using two kinds of markers of high molecular weight side and low molecular weight side. The molecular weight marker to be used in this case is DNA or RNA, which are selected based on the number of base thereof. That is, when electrophoresis of DNA or RNA is carried out, molecular weight of the analyte has been able to be obtained from positional relations with two molecular weight markers by setting positions of the molecular weight markers accurately. However, the molecular weight marker to be used here is DNA, RNA, or the like which is labeled with an intercalator, and had such problems that stability was poor; the molecular weight marker reacted with a substance in a sample when a serum sample was used; background was increased; the molecular weight marker showed a broad peak; and the like.
On the other hand, when an analyte is a protein or a compound, since mobility of the analyte varies depending on molecular weight or electric charge of the analyte, usually only one kind of internal standard substance is used and purpose of the use was to specify an approximate position of the analyte. Therefore, in such case when a protein or a compound is measured, development of a measurement method which does not have such problems in the measurement of DNA or RNA as described above and is capable of specifying a protein or a compound with good accuracy has been presently demanded.
In particular, when electrophoresis is carried out using a fine capillary as in microchip electrophoresis, there has been such a problem that migration time of a substance could vary even under the same conditions if chip is changed to another one, because of the fineness of the capillary. For this reason, it was sometimes difficult to specify the target peak. Further, since a high sensitivity measurement becomes possible in the case of such measurement method, it was essential to use an internal standard substance which does not influence on the measurement and is capable of measuring with good sensitivity. Therefore, development of a measurement method which can specify a peak of a measurement substance under the conditions as described above has been also demanded.